Method for producing a solar power receiving tube and resulting tube

ABSTRACT

The invention relates to a method for producing a solar power receiving tube and to the resulting tube, which is of the type that includes: an outer glass tube, an inner metal absorber through which a heat-transfer fluid flows, and an intermediate area in which the vacuum is produced. The method comprises the following steps:
         i. Production of the metal tubes   ii. Production of the glass tubes: namely a longer central glass tube and two shorter glass tubes for the ends.   iii. Process for the production of the Kovar rings or glass-metal transition elements   iv. Process for the welding of the Kovar rings to the tubes   v. Process for the production of the bellows or expansion compensating devices assemblies   vi. Assembly of the products obtained in the preceding operations   vii. Creation of the vacuum and anodising of the welds

TECHNICAL FIELD OF THE INVENTION

The present invention aims to present the sequence of assembly of thedifferent individual components that make up a solar power receivingtube and the resulting tube.

BACKGROUND OF THE INVENTION

The general principle of solar thermal technology is based on theconcept of the concentration of solar radiation to produce generallysteam, which is then used in standard power plants.

Solar energy collection, which has a relatively low density, is one ofthe greatest challenges in the development of solar thermal powerplants. There are two types of solar concentrators: linear concentratorsand point concentrators. Linear concentration is easier to install sinceit has fewer degrees of freedom, but it has a lower concentration factorand therefore it can reach lower temperatures than point concentrationtechnology.

That is why an effort is made in order to progress in the development ofthe receiving tubes used in linear concentration, to try to increase theconcentration factor achieved therein and to reduce losses, such thatthe overall performance of the concentration plant increases.

The invention before us explains the procedure of assembly of one ofthese receiving tubes of improved technology with respect to what existsin the state of the art, as well as the resulting tube.

In general, a receiving tube consists of two concentric tubes betweenwhich the vacuum is produced. The inner tube, through which the fluidthat is heated flows, is made of metal and the outer tube is made ofglass, usually borosilicate.

Between the metal tube and the glass tube, at the ends of the tube,there will be installed a series of elements or devices each with aspecific function. The most common are: expansion compensating device orbellow, evaporable getter or vacuum detector, non-evaporable getter orvacuum-enhancing system and different connection elements necessarybetween these devices.

As for the background on this type of procedures none in particular areworth mention since, depending on the components of the specific tubethat is going to be manufactured, the assembly procedure shall beconducted in one way or another. As an example of tube, but that hasdifferent design elements and located in different places and not beingan object of this document to detail them, patent US 20070209658 bySCHOTT can be mentioned.

DESCRIPTION OF THE INVENTION

The invention consists of developing an efficient assembly procedure fora receiving tube of those used in linear solar concentrators, either forparabolic trough or Fresnel concentrators.

The main parts that comprise the receiving tube are:

-   -   Expansion compensating system or bellows.    -   Cover    -   Vessel    -   Glass/metal transition element or Kovar ring.    -   Non-evaporable loop getter and closure system    -   Evaporable getter with attachment clip    -   Borosilicate tubes.    -   Metal tube.

These are all individual elements with the exception of the expansioncompensating device assembly or bellows which brings together, in anindependent assembly process, the manufacture of all the expansioncompensating mechanical assembly, as well as the inclusion of theevaporable and non-evaporable getters and their associated supports.

For non-evaporable getters or vacuum-enhancing system, the supportconsists of a cable wherein the getter material pills are insertedthrough a drill made in the center. The cable has a closure with afemale part and a male part which will be joined when all the pills areinserted.

For the evaporable getter or vacuum detector, the support consists of aclip with a circular end wherein the evaporable getter material pillwill be introduced.

The complete manufacturing procedure of the receiving tube encompasses,therefore, the following steps:

-   -   i. Process for the production of the metal tubes (generally made        of steel)    -   ii. Process for the production of the glass tubes (there are        three, central with approximately 3.6 m in length and two for        the ends of approximately 0.2 m in length)    -   iii. Process for the production of the Kovar rings    -   iv. Process for the welding of the Kovar rings to the tubes    -   v. Process for the production of bellows assemblies (there are        two, one for each end of the tube). The steps to be followed to        mount this assembly will be detailed in the preferred        embodiment.    -   vi. Assembly of the preceding products    -   vii. Creation of the vacuum in the intermediate zone, between        the glass and the metal tube and applying anodiśing spray to the        welds.

Next and with help of the figures, a more detailed explanation of alland each one of the stages that each of the steps of the procedurecomprises will be provided.

DESCRIPTION OF THE DRAWINGS

To complete the description being made and with the object of helping toa better understanding of the invention, a set of drawings isaccompanied where, with illustrative character and without limitation,the following has been represented:

FIG. 1: Process for the production of bellows assemblies. Step 1.Product 1.

FIG. 2: Process for the production of bellows assemblies. Step 2.Product 2.

FIG. 3: Process for the production of bellows assemblies. Step 3.Product 3.

FIG. 4: Process for the production of bellows assemblies. Step 4.Product 4

FIG. 5: Process for the production of bellows assemblies. Step 5.Product 5

FIG. 6: Process for the production of bellows assemblies. Step 6.Product 6

FIG. 7: Process for the production of bellows assemblies. Step 7.Product 7

FIG. 8: Process for the production of bellows assemblies. Step 8.Product 8

FIG. 9: Process for the production of bellows assemblies. Step 9.Product 9

FIG. 10: Attachment clamps

FIG. 11: End of the complete tube with olive for producing vacuum

FIG. 12: Perspective view of the complete receiving tube.

LIST OF REFERENCES

(1) Expansion compensating device or bellow

(2) Vessel

(3) Cover

(4) Bridge of fixation

(5) Clip-type support of evaporable getter

(6) Evaporable getter pill

(7) Closure of the of the non-evaporable getter, female part; (7′) malepart

(8) Non-evaporable getter pills

(9) Non-evaporable getter cable

(10) Attachment clamps

(11) Olive to create vacuum

PREFERRED EMBODIMENT OF THE INVENTION

To achieve a better understanding of the invention, the manufacturingprocedure according to a preferred embodiment will be described next.

As stated earlier, the procedure includes a series of main steps (i-vii)the respective stages of which will be studied.

-   i. Process for the production of the metal tubes

In a preferred embodiment these are tubes of steel which are polished,cleaned and a selective coating is applied by the sputtering technique.

The steel tube is heated so that it acquires an outer oxide layer onwhich the different layers will be applied.

-   ii. Process for the production of the glass tubes

The glass tube as a whole is composed of three borosilicate tubes: thecentral one with a length of 3.6 m and two for the ends with a length of0.2 m. One of the tubes of the ends will have in addition an olive forthe creation of the vacuum by pump.

-   iii. Process for the production of the Kovar rings

The Kovar rings are the transition element for carrying out the bindingof steel with glass, since it is an alloy of iron, cobalt and nickelwhich has a coefficient of thermal expansion very close to that of theglass.

-   iv. Process for the welding of the Kovar rings to the tubes.

Once received the three glass tubes, these can be cleaned in this momentand are welded to the Kovar piece in the following way:

-   -   Welding the first glass tube with a length of 0.2 m to the Kovar        piece by one of its ends (glass-metal welding).    -   Welding the second glass tube with a length of 0.2 m to the        Kovar piece by one of its ends (glass-metal welding).

Then a glass-glass welding is done between the free ends of the Kovarpiece of the smaller length tubes and the two ends of the tube of 3.6 min length.

Once the previous process is completed, the tube is taken to the sol-gelstation (anti-reflective coating) where the tube is cleaned throughwashing with water or ultrasound, it is dried and it is immersed in avat for the application of the anti-reflective. Next it is introduced inan oven with the purpose of consolidating the sol-gel layer andreleasing the stress of the glass due to the welding. This heattreatment for releasing stress could also be carried out just aftercarrying out the glass-metal welding and before taking the tube to thesol-gel station.

Then the temperature of the tube is reduced and a hydrophobic treatmentis provided for achieving a water-repellent surface.

Next the tube is dried with heat or air.

-   v. Process for the production of bellows assemblies

These are the expansion compensating devices (1) which arebellows-shaped plus all the elements that are bounded to them, such asthe evaporable getter and non-evaporable getters with their supports.

For the evaporable getter the support consists of a clip-type support(5) with a circular end wherein the evaporable getter material pill (6)will be introduced.

For the non-evaporable getters the support consists of a cable (9)wherein the getter material pills (8) are introduced through a drillmade in the center and the cable has a closure with a female part (7)and a male part (7′) that will be joined in the moment that all thepills (8) are in the cable.

Following the figures consecutively from FIG. 1 to FIG. 12, the steps ofthis process are:

-   -   Step 1: Bellows welding (1)+vessel (2)→Product 1    -   Step 2: Cover welding (3)+Product 1→Product 2    -   Step 3: Welding bridge of fixation (4) of the evaporable getter        at the bottom of Product 2→Product 3    -   Step 4: Introducing evaporable getter pill (6) in the dip-type        support (5)→Product 4    -   Step 5: Assembling Product 4 in product 3→Product 5    -   Step 6: Cable Assembly (9)+female part of the closure (7) of the        non-evaporable getter cable→Product 6    -   Step 7: Product 6 Assembly+Non-evaporable getter pills        (8)→Product 7    -   Step 8: Product 7 Assembly+male part of the closure (7′) of the        non-evaporable getter cable→Product 8    -   Step 9: Product 5 Assembly+Product 8→Product 9    -   Step 10: Assembly of the attachment clamps (10) of the        non-evaporable getter→Product 10 or whole assembly

-   vi. Assembly of the preceding assemblies, with the following    options:

-   1) The metal absorber tube is located inside the glass tube and then    the bellow assembly is welded at both ends (product 10). For this,    the vessel (2) is bound by welding with the wall of the metal    absorber tube (bellow assembly or product 10 binding to the absorber    tube) and the cover (3) with the Kovar ring (bellow assembly or    product 10 binding to the glass tube).

-   2) Binding of the bellow assembly or product 10 by welding the cover    (3) to the Kovar ring of the glass tube at both ends of the tube,    then the metal absorber tube is introduced inside the assembly by    any of the ends and by both ends the part of the vessel (2) of the    bellow assembly or product 10 is welded to the wall of the metal    absorber tube.

-   3) At one end of the glass tube the bellow assembly or product 10 is    welded by its cover (3) to the Kovar ring and then the metal    absorber tube is introduced by the other end, the metal absorber    tube is welded to the cover (3) of the bellow assembly or product 10    of the first end and then by the other end of the glass tube the    vessel (2) and absorber tube, cover (3) and Kovar ring of the glass    tube are welded.

-   4) The bellow assembly or product 10 is welded by its vessel (2) to    a first end of the metal absorber tube, the glass tube is introduced    by the other end and the cover (3) of the bellow assembly or product    10 is welded to the Kovar ring of the first end of the glass. The    bellow assembly or product 10 is placed on the other end and vessel    (2) and absorber tube cover (3) and Kovar ring are welded.

Note: In any of the assembly processes described above is desirable,though not essential, to perform a preload of the bellow assembly beforecarrying out the welding of said assembly with the metal tube. Thispreload involves a series of advantages, such as the fact that it fixespossible manufacturing deviations, allows to place the expansioncompensating device in a position that prepares it to receivetensile-compressive loads, it can be used to pretension the glass tubeand ensure a smaller deflection during operation, as well as itguarantees a better performance in exceptional circumstances ofassembly, transport, etc.

To perform such a preload it is noted that all the described assemblyprocesses have a welding step of the bellow assembly to the metal tubethat is carried out, by welding the two bellows to the metal tubesimultaneously or by welding first one side and then the other. Thislast way of proceeding has the drawback that, in order to introduce thepreload in the bellows, all the preload corresponding to the two bellowshas to be introduced first in one of them (which means that bellowsupports a preload twice the nominal) and then proceeding to the weldingof the other bellow, such that until the second bellow is not placed thepreload is not distributed between both of them. Thus, in the case ofintroducing preload in the bellows, it would be convenient to use theassemblies where the positions of the bellows assemblies are coordinatedon the metal tube independently among them, so as to not submit thebellows to an excessive load.

-   vii. Creates the vacuum in the intermediate area, between the glass    tube and the metal tube and applying anodizing spray to the welds.    FIG. 11 shows an olive (11) for the creation of that vacuum.

With this the final product, the receiving tube, prepared for itsassembly in a solar concentration plant is obtained.

This system is especially designed for its application in the assemblyof solar technology receiving tubes, but it does not preclude itsextension to other areas of the industry that require similar features.

1. Method for producing a solar power receiving tube of those comprisingan outer glass tube, an inner metal absorber tube through which theheat-transfer fluid flows, and an intermediate area in which the vacuumis produced comprising the following steps: i. Production of the metaltubes. ii. Production of the glass tubes: namely a longer central glasstube and two shorter glass tubes for the ends. iii. Process for theproduction of the Kovar rings or glass-metal transition elements. iv.Process for the welding of the Kovar rings to the tubes. v. Process forthe production of the bellows or expansion compensating devicesassemblies. vi. Assembly of the products obtained in the precedingoperations. vii. Creation of the vacuum and anodising of the welds. 2.Method for producing a solar power receiving tube according to claim 1characterized In that the process for the production of the metal tubesinvolves the following stages: Polishing of the tube Cleaning of thetube Selective coating through the sputtering technique Heating of themetal tube so that it acquires an outer oxide layer on which thedifferent layers will be applied.
 3. Method for producing a solar powerreceiving tube according to claim 1 characterized in that one of theglass tubes of the ends will have an olive (11) for the creation of thevacuum by pump.
 4. Method for producing a solar power receiving tubeaccording to claim 1 characterized In that Kovar rings are manufacturedas an iron, cobalt and nickel alloy with coefficient of thermalexpansion very close to that of the glass.
 5. Method for producing asolar power receiving tube according to claim 4 characterized in thatthe glass tubes are welded to the. Kovar piece in the following mannerWelding the first shorter glass tube of one end to the Kovar piece byone of its ends (glass-metal welding). Welding the second shorter glasstube of the other end to the Kovar piece by one of its ends (glass-metalwelding). Glass-glass welding between the free ends of the Kovar pieceof the smaller length tubes and the two ends of the longer glass tube.6. Method for producing a solar power receiving tube according to claim5 characterized in that once the glass tubes are welded to the Kovarrings they are taken to the sol-gel station. (anti-reflective coating)where the tube is cleaned through washing with water or ultrasound, itis dried and it is immersed in a vat for the application of theanti-reflective. Next it is introduced in an oven with the purpose ofconsolidating the sol-gel layer and releasing the stress of the glassdue to the welding. This heat treatment for releasing stress could alsobe carried out just after carrying out the glass-metal welding andbefore taking the tube to the sol-gel station. Then the temperature ofthe tube is reduced and a hydrophobic treatment is provided forachieving a water-repellent surface. Finally the tube is dried with heator air.
 7. Method for producing a solar power receiving tube accordingto claim 6 characterized in that the heat treatment for releasing stresscould be carried out after carrying out the glass-metal welding andbefore taking the tube to the sol-gel station.
 8. Method for producing asolar power receiving tube according to claim 1 characterized in thatthe bellows assemblies or expansion compensating devices comprise thebellows-shaped expansion compensating devices (1), the vessel (2), thecover (3), the evaporable getter (6) with its support (5) and thenon-evaporable getters (8) with their support. The support of thenon-evaporable getters consists of a cable (9) wherein the gettermaterial pills (8) are introduced and the cable (9) has a closure with afemale part (7) and a male part (7′) that will be joined in the momentthat all the pills (8) are in the cable (9). For the non-evaporablegetter the support consists of a clip (5) with a circular end whereinthe evaporable getter material pill (6) will be introduced.
 9. Methodfor producing a solar power receiving tube according to claim 8characterized in that the process for the production of the expansioncompensating devices or Bellows involves the following steps: Step 1:Bellows welding (1)+Vessel (2)→Product 1 Step 2: Cover welding(3)+Product 1→Product 2 Step 3: Welding bridge of fixation (4) of theEvaporable getter at the bottom of the Product 2→Product 3 Step 4:Introducing the evaporable getter pill (6) in the dip-type support(5)→Product 4 Step 5: Assembling Product 4 in the product 3→Product 5Step 6: Cable Assembly (9)+female part of the closure (7) of thenon-evaporable getter cable Product 6 Step 7: Assembly product6+Non-evaporable getter pills (8)→Product 7 Step 8: Assembly Product7+male part of the closure (7′) of the non-evaporable getter cable+Product 8 Step 9: Assembly Product 5+Product 8→Product 9 Step 10:assembly of the attachment clamps (10)→Product 10 or whole assembly. 10.Method for producing a solar power receiving tube according to claims1-9 characterized in that there is a first option for the assembly ofthe preceding assemblies consisting of locating the metal absorber tubeinside the glass tube and then the bellow assembly is welded at bothends (product 10). For this, the vessel (2) is bound by welding with thewall of the metal absorber tube (bellow assembly or product 10 bindingto the absorber tube) and the cover (3) with the Kovar ring (bellowassembly or product 10 binding to the glass tube).
 11. Method forproducing a solar power receiving tube according to claims 1-9characterized in that there is a second option for the assembly of thepreceding assemblies consisting of carrying out the binding of thebellow assembly or product 10 by welding the cover (3) to the Kovar ringof the glass tube at both ends of the tube, then the metal absorber tubeis introduced inside the assembly by any of the ends and by both endsthe part of the vessel (2) of the bellow assembly or product 10 iswelded to the wall of the metal absorber tube.
 12. Method for producinga solar power receiving tube according to claims 1-9 characterized inthat there is a third option for the assembly of the precedingassemblies consisting of welding at one end of the glass tube the bellowassembly or product 10 by its cover (3) to the Kovar ring and then themetal absorber tube is introduced by the other end, the metal absorbertube is welded to the cover (3) of the bellow assembly or product 10 ofthe first end and then by the other end of the glass tube the vessel (2)and absorber tube, cover (3) and Kovar ring of the glass tube arewelded.
 13. Method for producing a solar power receiving tube accordingto claims 1-9 characterized in that there is a fourth option for theassembly of the preceding assemblies consisting of welding the bellowassembly or product 10 by its vessel (2) to a first end of the metalabsorber tube, the glass tube is introduced by the other end and thecover (3) of the bellow assembly or product 10 is welded to the Kovarring of the first end of the glass. The bellow assembly or product 10 isplaced on the other end and vessel (2) and absorber tube cover (3) andKovar ring are welded.
 14. Method for producing a solar power receivingtube according to claims 10-13 characterized in that a preload step ofthe bellows assemblies or expansion compensating devices is sandwiched,before welding said assemblies to the metal tube.
 15. Solar powerreceiving tube of those comprising an outer glass tube, an inner metalabsorber tube through which the heat-transfer fluid flows and anintermediate vacuum area characterized In that it has been manufacturedaccording to the method of manufacture described in the previous claims.